A) A = ( A + B ) (2 ≤ A + B ) (2+ A − B )2 = 2A 2B 2 + ≤ . ==...

139. a) ^{A =} ( ^{a + b} ) (

^{≤ a + b} ) (

^{+ a − b} )

^{= 2a 2b 2 + ≤ .}

 =

= ⇔   ⇔ = =

1

a b

max A 2 a b

a b 1 2

 + =

b) Ta cĩ : ( ^{a + b} ) (

^{≤ a + b} ) (

^{+ a − b} )

^{= 2(a}

^{+ b}

^{+ 6ab)}

( ) ( )

+ ≤ + + + ≤ + +

a c 2(a c 6ac) ; a d 2(a d 6ad)

b c 2(b c 6bc) ; b d 2(b d 6bd)

Tương tự :

( )

+ ≤ + +

c d 2(c d 6cd)

Suy ra : B ≤ 6(a

+ b

+ c

+ d

+ 2ab + 2ac + 2ad + 2bc + 2bd + 2cd) = 6(a + b + c + d)

≤ 6

 = = =

= ⇔    + + + = ⇔ = = = =

a b c d

max B 6 a b c d

a b c d 1 4